As landing aid radio means, both for manual landing and for automatic landing, various radionavigation systems are known, in particular a microwave landing system of MLS type or an instrument landing system of ILS type. The operational requirements make it necessary to employ an MLS type or ILS type system which makes it possible to perform landings regardless of the climatic conditions and to comply with the demands imposed by international standards.
It is known that the main constraints that are related to the types of approach and to the constraints of installing the antennas on an aircraft are the following constraints:                a so-called 19-feet (or 5.8-meter) rule in the final phase of the approach (representing the vertical deviation between the trajectory passing through the reception antenna and the trajectory passing beneath the main undercarriage) which is a demand imposed by international standards;        the shape of the radiation patterns of the antennas and the potential masking of the antennas possibly related to fixed or moving obstacles such as the undercarriages, but also to the attitudes of the aircraft throughout the approach phase; and        the deviation between the heading of the aircraft and the heading of the runway, which is significant at the start of the approach.        
To satisfy all the aforesaid constraints during a landing, it is primarily necessary to install an antenna on the upper part of the aircraft, so as to guarantee the reception of a radio signal during an approach with significant angles of capture of the axis of the runway (this signal is used to compute the position of the airplane with respect to the approach axis). Moreover, to comply with the 19-feet rule, it is necessary to use an antenna installed on the lower part of the aircraft so as to have a vertical deviation, between the trajectory passing through this antenna and that passing through the lowest point of the main undercarriage, which is less than 19 feet (or around 5.8 meters).
To comply with the above two constraints, the installation becomes complex or even impossible for aircraft, the diameter and the length of whose fuselage are significant.
In addition to the aforesaid constraints related to the installation of the antennas on the aircraft, there are also constraints related to the radionavigation receivers. A first constraint is that an “MLS” type system for example works with angular values and a second constraint is that in order to be able to correct, on the basis of a fixed and constant metric value, the vertical position of the aircraft, the receiver must necessarily know the position of the aircraft with respect to the threshold of the runway, so as to transform a metric correction into an angular correction.